V4l2的官网例程代码：
/* * V4L2 video capture example * * This program can be used and distributed without restrictions. */#include <stdio.h>#include <stdlib.h>#include <string.h>#include <assert.h>#include <getopt.h> /* getopt_long() */#include <fcntl.h> /* low-level i/o */#include <unistd.h>#include <errno.h>#include <malloc.h>#include <sys/stat.h>#include <sys/types.h>#include <sys/time.h>#include <sys/mman.h>#include <sys/ioctl.h>#include <asm/types.h> /* for videodev2.h */#include <linux/videodev2.h>#define CLEAR(x) memset (&(x), 0, sizeof (x))typedef enum { IO_METHOD_READ, IO_METHOD_MMAP, IO_METHOD_USERPTR,} io_method;struct buffer { void * start; size_t length;};static char * dev_name = NULL;static io_method io = IO_METHOD_MMAP;static int fd = -1;struct buffer * buffers = NULL;static unsigned int n_buffers = 0;FILE *fp;char *filename = "test.yuv\0";static void errno_exit(const char * s) { fprintf(stderr, "%s error %d, %s/n", s, errno, strerror(errno)); exit(EXIT_FAILURE);}static int xioctl(int fd, int request, void * arg) { int r; do { r = ioctl(fd, request, arg); } while (-1 == r && EINTR == errno); return r;}static void process_image(const void * p, int size) { fwrite(p, size, 1, fp);}static int read_frame(void) { struct v4l2_buffer buf; unsigned int i; switch (io) { case IO_METHOD_READ: if (-1 == read(fd, buffers[0].start, buffers[0].length)) { switch (errno) { case EAGAIN: return 0; case EIO: /* Could ignore EIO, see spec. */ /* fall through */ default: errno_exit("read"); } } process_image(buffers[0].start, buffers[0].length); break; case IO_METHOD_MMAP: CLEAR(buf); buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; buf.memory = V4L2_MEMORY_MMAP; if (-1 == xioctl(fd, VIDIOC_DQBUF, &buf)) { switch (errno) { case EAGAIN: return 0; case EIO: /* Could ignore EIO, see spec. */ /* fall through */ default: errno_exit("VIDIOC_DQBUF"); } } assert(buf.index < n_buffers); process_image(buffers[buf.index].start, buf.length); if (-1 == xioctl(fd, VIDIOC_QBUF, &buf)) errno_exit("VIDIOC_QBUF"); break; case IO_METHOD_USERPTR: CLEAR(buf); buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; buf.memory = V4L2_MEMORY_USERPTR; if (-1 == xioctl(fd, VIDIOC_DQBUF, &buf)) { switch (errno) { case EAGAIN: return 0; case EIO: /* Could ignore EIO, see spec. */ /* fall through */ default: errno_exit("VIDIOC_DQBUF"); } } for (i = 0; i < n_buffers; ++i) if (buf.m.userptr == (unsigned long) buffers[i].start && buf.length == buffers[i].length) break; assert(i < n_buffers); process_image((void *) buf.m.userptr, buf.length); if (-1 == xioctl(fd, VIDIOC_QBUF, &buf)) errno_exit("VIDIOC_QBUF"); break; } return 1;}static void mainloop(void) { unsigned int count; count = 100; while (count-- > 0) { for (;;) { fd_set fds; struct timeval tv; int r; FD_ZERO(&fds); FD_SET(fd, &fds); /* Timeout. */ tv.tv_sec = 2; tv.tv_usec = 0; r = select(fd + 1, &fds, NULL, NULL, &tv); if (-1 == r) { if (EINTR == errno) continue; errno_exit("select"); } if (0 == r) { fprintf(stderr, "select timeout/n"); exit(EXIT_FAILURE); } if (read_frame()) break; /* EAGAIN - continue select loop. */ } }}static void stop_capturing(void) { enum v4l2_buf_type type; switch (io) { case IO_METHOD_READ: /* Nothing to do. */ break; case IO_METHOD_MMAP: case IO_METHOD_USERPTR: type = V4L2_BUF_TYPE_VIDEO_CAPTURE; if (-1 == xioctl(fd, VIDIOC_STREAMOFF, &type)) errno_exit("VIDIOC_STREAMOFF"); break; }}static void start_capturing(void) { unsigned int i; enum v4l2_buf_type type; switch (io) { case IO_METHOD_READ: /* Nothing to do. */ break; case IO_METHOD_MMAP: for (i = 0; i < n_buffers; ++i) { struct v4l2_buffer buf; CLEAR(buf); buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; buf.memory = V4L2_MEMORY_MMAP; buf.index = i; if (-1 == xioctl(fd, VIDIOC_QBUF, &buf)) errno_exit("VIDIOC_QBUF"); } type = V4L2_BUF_TYPE_VIDEO_CAPTURE; if (-1 == xioctl(fd, VIDIOC_STREAMON, &type)) errno_exit("VIDIOC_STREAMON"); break; case IO_METHOD_USERPTR: for (i = 0; i < n_buffers; ++i) { struct v4l2_buffer buf; CLEAR(buf); buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; buf.memory = V4L2_MEMORY_USERPTR; buf.index = i; buf.m.userptr = (unsigned long) buffers[i].start; buf.length = buffers[i].length; if (-1 == xioctl(fd, VIDIOC_QBUF, &buf)) errno_exit("VIDIOC_QBUF"); } type = V4L2_BUF_TYPE_VIDEO_CAPTURE; if (-1 == xioctl(fd, VIDIOC_STREAMON, &type)) errno_exit("VIDIOC_STREAMON"); break; }}static void uninit_device(void) { unsigned int i; switch (io) { case IO_METHOD_READ: free(buffers[0].start); break; case IO_METHOD_MMAP: for (i = 0; i < n_buffers; ++i) if (-1 == munmap(buffers[i].start, buffers[i].length)) errno_exit("munmap"); break; case IO_METHOD_USERPTR: for (i = 0; i < n_buffers; ++i) free(buffers[i].start); break; } free(buffers);}static void init_read(unsigned int buffer_size) { buffers = calloc(1, sizeof(*buffers)); if (!buffers) { fprintf(stderr, "Out of memory/n"); exit(EXIT_FAILURE); } buffers[0].length = buffer_size; buffers[0].start = malloc(buffer_size); if (!buffers[0].start) { fprintf(stderr, "Out of memory/n"); exit(EXIT_FAILURE); }}static void init_mmap(void) { struct v4l2_requestbuffers req; CLEAR(req); req.count = 4; req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; req.memory = V4L2_MEMORY_MMAP; if (-1 == xioctl(fd, VIDIOC_REQBUFS, &req)) { if (EINVAL == errno) { fprintf(stderr, "%s does not support " "memory mapping/n", dev_name); exit(EXIT_FAILURE); } else { errno_exit("VIDIOC_REQBUFS"); } } if (req.count < 2) { fprintf(stderr, "Insufficient buffer memory on %s/n", dev_name); exit(EXIT_FAILURE); } buffers = calloc(req.count, sizeof(*buffers)); if (!buffers) { fprintf(stderr, "Out of memory/n"); exit(EXIT_FAILURE); } for (n_buffers = 0; n_buffers < req.count; ++n_buffers) { struct v4l2_buffer buf; CLEAR(buf); buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; buf.memory = V4L2_MEMORY_MMAP; buf.index = n_buffers; if (-1 == xioctl(fd, VIDIOC_QUERYBUF, &buf)) errno_exit("VIDIOC_QUERYBUF"); buffers[n_buffers].length = buf.length; buffers[n_buffers].start = mmap(NULL /* start anywhere */, buf.length, PROT_READ | PROT_WRITE /* required */, MAP_SHARED /* recommended */, fd, buf.m.offset); if (MAP_FAILED == buffers[n_buffers].start) errno_exit("mmap"); }}static void init_userp(unsigned int buffer_size) { struct v4l2_requestbuffers req; unsigned int page_size; page_size = getpagesize(); buffer_size = (buffer_size + page_size - 1) & ~(page_size - 1); CLEAR(req); req.count = 4; req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; req.memory = V4L2_MEMORY_USERPTR; if (-1 == xioctl(fd, VIDIOC_REQBUFS, &req)) { if (EINVAL == errno) { fprintf(stderr, "%s does not support " "user pointer i/o/n", dev_name); exit(EXIT_FAILURE); } else { errno_exit("VIDIOC_REQBUFS"); } } buffers = calloc(4, sizeof(*buffers)); if (!buffers) { fprintf(stderr, "Out of memory/n"); exit(EXIT_FAILURE); } for (n_buffers = 0; n_buffers < 4; ++n_buffers) { buffers[n_buffers].length = buffer_size; buffers[n_buffers].start = memalign(/* boundary */page_size, buffer_size); if (!buffers[n_buffers].start) { fprintf(stderr, "Out of memory/n"); exit(EXIT_FAILURE); } }}static void init_device(void) { struct v4l2_capability cap; struct v4l2_cropcap cropcap; struct v4l2_crop crop; struct v4l2_format fmt; unsigned int min; if (-1 == xioctl(fd, VIDIOC_QUERYCAP, &cap)) { if (EINVAL == errno) { fprintf(stderr, "%s is no V4L2 device/n", dev_name); exit(EXIT_FAILURE); } else { errno_exit("VIDIOC_QUERYCAP"); } } if (!(cap.capabilities & V4L2_CAP_VIDEO_CAPTURE)) { fprintf(stderr, "%s is no video capture device/n", dev_name); exit(EXIT_FAILURE); } switch (io) { case IO_METHOD_READ: if (!(cap.capabilities & V4L2_CAP_READWRITE)) { fprintf(stderr, "%s does not support read i/o/n", dev_name); exit(EXIT_FAILURE); } break; case IO_METHOD_MMAP: case IO_METHOD_USERPTR: if (!(cap.capabilities & V4L2_CAP_STREAMING)) { fprintf(stderr, "%s does not support streaming i/o/n", dev_name); exit(EXIT_FAILURE); } break; } /* Select video input, video standard and tune here. */ CLEAR(cropcap); cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; if (0 == xioctl(fd, VIDIOC_CROPCAP, &cropcap)) { crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; crop.c = cropcap.defrect; /* reset to default */ if (-1 == xioctl(fd, VIDIOC_S_CROP, &crop)) { switch (errno) { case EINVAL: /* Cropping not supported. */ break; default: /* Errors ignored. */ break; } } } else { /* Errors ignored. */ } CLEAR(fmt); fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; fmt.fmt.pix.width = 640; fmt.fmt.pix.height = 480; fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV; fmt.fmt.pix.field = V4L2_FIELD_INTERLACED; if (-1 == xioctl(fd, VIDIOC_S_FMT, &fmt)) errno_exit("VIDIOC_S_FMT"); /* Note VIDIOC_S_FMT may change width and height. */ /* Buggy driver paranoia. */ min = fmt.fmt.pix.width * 2; if (fmt.fmt.pix.bytesperline < min) fmt.fmt.pix.bytesperline = min; min = fmt.fmt.pix.bytesperline * fmt.fmt.pix.height; if (fmt.fmt.pix.sizeimage < min) fmt.fmt.pix.sizeimage = min; switch (io) { case IO_METHOD_READ: init_read(fmt.fmt.pix.sizeimage); break; case IO_METHOD_MMAP: init_mmap(); break; case IO_METHOD_USERPTR: init_userp(fmt.fmt.pix.sizeimage); break; }}static void close_device(void) { if (-1 == close(fd)) errno_exit("close"); fd = -1;}static void open_device(void) { struct stat st; if (-1 == stat(dev_name, &st)) { fprintf(stderr, "Cannot identify '%s': %d, %s/n", dev_name, errno, strerror(errno)); exit(EXIT_FAILURE); } if (!S_ISCHR(st.st_mode)) { fprintf(stderr, "%s is no device/n", dev_name); exit(EXIT_FAILURE); } fd = open(dev_name, O_RDWR /* required */| O_NONBLOCK, 0); if (-1 == fd) { fprintf(stderr, "Cannot open '%s': %d, %s/n", dev_name, errno, strerror(errno)); exit(EXIT_FAILURE); }}static void usage(FILE * fp, int argc, char ** argv) { fprintf(fp, "Usage: %s [options]/n/n" "Options:/n" "-d | --device name Video device name [/dev/video]/n" "-h | --help Print this message/n" "-m | --mmap Use memory mapped buffers/n" "-r | --read Use read() calls/n" "-u | --userp Use application allocated buffers/n" "", argv[0]);}static const char short_options[] = "d:hmru";static const struct option long_options[] = { { "device", required_argument, NULL, 'd' }, { "help", no_argument, NULL, 'h' }, { "mmap", no_argument, NULL, 'm' }, { "read", no_argument, NULL, 'r' }, { "userp", no_argument, NULL, 'u' }, { 0, 0, 0, 0 } };int main(int argc, char ** argv) { dev_name = "/dev/video0"; for (;;) { int index; int c; c = getopt_long(argc, argv, short_options, long_options, &index); if (-1 == c) break; switch (c) { case 0: /* getopt_long() flag */ break; case 'd': dev_name = optarg; break; case 'h': usage(stdout, argc, argv); exit(EXIT_SUCCESS); case 'm': io = IO_METHOD_MMAP; break; case 'r': io = IO_METHOD_READ; break; case 'u': io = IO_METHOD_USERPTR; break; default: usage(stderr, argc, argv); exit(EXIT_FAILURE); } } open_device(); init_device(); start_capturing(); fp = fopen(filename, "wa+"); mainloop(); fclose(fp); stop_capturing(); uninit_device(); close_device(); exit(EXIT_SUCCESS); return 0;}另外补充一个常用的V4L2编程流程图
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